A representative feature extraction technique to extract from a data group formed of a multiplicity of data items a feature representative of the data group is used in the wide fields of image processing, image recognition, data mining, etc. For example, an image can be compressed by extracting from image data a representative feature well representing the characteristics of the whole or a portion of the image and by expressing the image as the representative feature and a difference from the representative feature. The technique can be also applied to sea rescue or the like; a drowning person, a person drifting on the sea, a driftage or the like can be detected by extracting, as a representative feature, pixels representative of the pixel values of the sea surface from an image of the sea surface taken from high above the sea, and by detecting pixels different from the representative feature. Further, technique can also be applied to behavior mining or the like, i.e., analysis of a behavior using a representative feature, which is performed by dividing a continuous behavior pattern into partial patterns so as to form similar behavior patterns and by extracting from the partial patterns features representative of the partial patterns.
In an image processing apparatus described in JP2007-142883A [PL1], as an example of an apparatus performing representative feature extraction, motion vectors are divided into groups similar in motion and an average vector is determined with respect to each group, thereby determining representative vectors representative of the respective groups. In an image region dividing method described in JP11-167634A [PL2], RGB values of pixels of an image are combined to form a histogram, thereby dividing the image into regions similar in color. An average color or a most frequent color is obtained as a representative color with respect to each region. In an image processing apparatus described in JP2008-065803A [PL3], maximum values in a histogram in a color space are selected to obtain representative colors, and expression in the representative colors is performed. In this way, color limiting processing is performed to reduce the number of colors used in an input image.
As a technique relating to the present invention, JP2002-222419A [PL4] discloses a technique to divide an image into regions or clusters. In a moving picture coding apparatus described in JP2005-260588A [PL5], the sum of the absolute values of the differences in the pixel values is obtained on a pixel-by-pixel basis between each frame in an interval and the other frames in the interval to select a representative frame in the interval. The total sum of such sums with respect to all the other frames is defined as a disparity value, and the frame having the smallest disparity value is obtained as a representative frame in the interval. JP6-209469A [PL6] discloses hierarchically performing coding on image data.